It is known to provide an image sensor that can scale an image, that is to re-size the image. The image data is passed through a decimating digital filter to produce a reduced size image, for efficient display on small resolution screens or for use in applications such as a viewfinder mode of a video recorder, digital still camera (DSC), or an image sensor in general. The lesser amount of data that is required for each scaled image (as compared with the amount of data required for the full field of view image) also reduces data transfer and reduces processor power consumption. An increased frame rate can also be achieved.
The scaling is enabled by line memories associated with the image sensor. The line memories are used to combine pixels from successive rows for the reconstruction of the reduced resolution image. In a color image sensor, the line memories take into account the pattern of a color filter array (CFA), in which each row comprises information on only two out of three colors in the array. A common type of CFA is a Bayer array, in which one row comprises red and green filters, while an adjacent row comprises green and blue filters.
A scaling function is useful for various applications as mentioned above. Indeed, in the field of image sensors for mobile phones, a Standard Mobile Imaging Architecture (SMIA) has been proposed which defines specifications for mobile imaging functionality, mechanics, reliability, performance characterization, and interfaces. SMIA profile 2 specifies a mandatory image scaling function, and therefore imposes a requirement for line memories for performing the scaling.
The defectivity content of a viewed image can be improved by the use of digital filters. One of the simplest is a median filter applied to a kernel of three pixel values. This filter is a moving averager. As illustrated in FIG. 1, the central value (Cx) is replaced by the average of the two adjacent values, C1 and C2. This filter moves “on the fly” along the pixel rows.
The median filter is implemented by the simple addition of registers, the required number of which depends on the image sensor system's processing power. Because no line memory is required, little area is used. However, the median filter is destructive of high frequency data. For example, in an image which includes black text on a white background which is imaged at a distance, the sharp contrast between successive dark and light portions is destroyed by the median filter, and the text appears blurred.
Given the increasing resolution and image quality requirements of sensors used for mobile imaging, there is a very high probability of major point defects due to statistical process variation, thus meaning that the simple median filter is increasingly not a practical option.
More sophisticated defect correction algorithms that give accurate results for high frequency data rely on the use of their own dedicated line memories. These line memories take up a large amount of space on the image sensor chip.